Phase modulation of a carrier signal is, of course, well known for transmitting information onto a carrier signal. A common practice in communications is to employ bi-phase shift keyed (BPSK) or quadrature phase shift keyed (QPSK) modulation of the carrier signal to impart information thereon. That is, a receiver may detect variations in phase of a received carrier signal for obtaining information on the carrier signal. Applications for such phase shift keyed modulators include, among others, satellite communications and terrestrial digital radio systems.
High frequency phase modulators, as used herein, refer to those associated with microwave and mm-wave phase shift keyed applications. High frequency phase modulators are particularly difficult to construct economically to meet the performance requirements of having essentially constant amplitude for all phase states, and substantially constant phase states over a narrow bandwidth. A common practice for constructing such phase modulators is the employment of modulation techniques at low frequencies, and then to up-convert or frequency translate the resulting modulation waveform to the higher frequency. Such practices involve extra mixing circuitry and necessitate complex circuitry. For microwave and mm-wave applications, such complex circuitry adds to the degradation of performance due to, among others, signal insertion loss and signal degradation.
A phase shift keyed modulator is illustrated in a paper entitled, "Embedded Transmission Line (ETL) MMIC for Low Cost, High-Density Wireless Communication Applications," by H. Q. Tserng, et.al., in 1997 IEEE Radio Frequency Integrated Circuits Symposium (0-7803-4603-9/97), incorporated herein by reference. In this paper, four pairs of transmission line segments are fabricated by way of MMIC technology. These line segments are switched in and out to provide a 16-state phase modulator for phased array radar applications. For radar applications, it is desired to a have a multi-bit phase shifter in which each phase state the phase is intended to be a linear function of frequency.
The phase shift keyed modulator of Tserng, et.al., although acceptable for radar applications, it is unacceptable for communication applications. This is so, since ideally it is not acceptable in communication applications for the phase to change with input frequency.
Therefore, there is a need for phase modulator for high frequency communication applications that is simple to construct, simple in structure, and provides good performance.
There is a need for a high frequency phase modulator which provides a quadrature-phase or bi-phase change which is substantially constant with frequency over a narrow bandwidth and which is simple to implement using MMIC integrated circuit fabrication technology.